1. Field of the Invention
This invention relates to a signal to interference power ratio measuring apparatus and a signal to interference power ratio measuring method as well as a transmission power controlling method under a CDMA communication system suitable for use to effect transmission power control of, for example, a mobile radio communication apparatus, particularly a mobile radio communication apparatus, which adopts CDMA (Code Division Multiple Access) which employs a multiple access method.
2. Description of the Related Art
In recent years, attention is paid to, from among transmission systems for use for radio communication, a code division multiple access (CDMA) system which is a multiple access system which makes use of a spread spectrum and exhibits a high frequency utilization efficiency. Particularly, in cellular DS/CDMA (Direct Sequence/Code Division Multiple Access) mobile communication, in order to increase the subscriber capacity while maintaining a required line quality, transmission power control which solves the remote/near problem is an important technique.
FIG. 9 shows a radio communication system to which an ordinary DS/CDMA communication system is applied. Referring to FIG. 9, the radio communication system 100 shown includes a base station 101 and a plurality of terminal stations (mobile stations) 102-1 to 102-n (n is a natural number equal to or greater than 2) so that information such as speech or data is transmitted from the single base station 101 to the plurality of terminal stations 102-1 to 102-n or vice versa.
More particularly, since the CDMA system multiplexes information using codes in order to transmit the information from the base station 101 to the plurality of terminal stations 102-1 to 102-n in FIG. 9, signals destined for all of the terminal stations 102-1 to 102-n can be transmitted at the same time with the same frequency.
One of methods for realizing the transmission power control described above in the radio communication system shown in FIG. 9 is closed loop transmission power control wherein signal to interference power ratios (SIRs) of received signals from the terminal stations 102-1 to 102-n are measured by the base station 101 and the transmission powers of the terminal stations 102-1 to 102-n are controlled so that the values thereof may be kept fixed.
Generally, as a signal to interference power ratio measuring method, a method is known wherein an average received power after RAKE composition (delayed wave composition) is regarded as a received power (S) and a diffusion of the received power is calculated as an interference power (I). It is to be noted that the RAKE composition mentioned above is performed for delayed waves as a plurality of received waves having different delay times after they are synchronized with one another and then inverse diffusion processing and transmission line channel estimation processing are performed.
FIG. 10 shows an SIR measuring apparatus which measures a signal to interference power ratio (SIR) using the technique described above. Referring to FIG. 10, the SIR measuring apparatus 80 shown includes a quadrant detection section 80A, a vector mean calculation section 80B, a square calculation section 80C, a mean square calculation section 80D, a subtraction section 80E, and an SIR calculation section 80F.
The quadrant detection section 80A detects a quadrant of a received signal vector after RAKE composition. In particular, the quadrant detection section 80A effects degeneracy of the received signal vector to a single quadrant by calculating absolute values of an in-phase component and an orthogonal component of the received signal vector individually and so forth.
The vector mean calculation section 80B calculates a vector mean of an output of the quadrant detection section 80A, and the square calculation section 80C calculates a received power (S) from the vector mean value from the vector mean calculation section 80B. The calculated received value (S) is outputted to the SIR calculation section 80F which is described below.
The mean square calculation section 80D calculates a mean square of the received signal after RAKE composition inputted thereto. The subtraction section 80E subtracts an output of the square calculation section 80C from an output of the mean square calculation section 80D to calculate a diffusion of the received signal. An output of the subtraction section 80E is used as an interference power (I).
The SIR calculation section 80F calculates an SIR (S/I ratio) based on an output (S: received power) of the square calculation section 80C and an output (I: interference power) of the subtraction section 80E. In this manner, in the SIR measuring apparatus 80 shown in FIG. 10, an SIR is calculated using a mean value of a received signal vector after RAKE composition calculated by the vector mean calculation section 80B for both of a received power and an interference power.
With the SIR measuring technique described above, however, the SIR measurement accuracy sometimes exhibits a deterioration under a fast fading environment or under an inter-station interference and noise environment. Therefore, the SIR measurement technique has a subject to be solved in that, where transmission power control is performed based on an SIR measured under such a circumstance as just described, such deterioration of the SIR measurement accuracy resultantly has an influence also upon the accuracy in transmission power control itself.
One of causes which deteriorate the accuracy in SIR measurement resides in that a mean value of a received signal vector is used for both of a received power (S) and an interference power (I). The received signal power is unstable because it is varied by a large amount by noise or by an estimation accuracy degree of a transmission line channel. Therefore, in order to assure a high degree of accuracy, it is necessary to calculate a mean value over a long period.
Further, since also use of a value in a data symbol interval in which the transmission line channel is not settled results in deterioration in accuracy, such a contrivance as to estimate a transmission line channel with a high degree of accuracy and to use, in order to assure a high degree of accuracy in SIR measurement, only a value within a pilot symbol interval is required.
Generally, a pilot symbol is arranged at a leading end and a trailing end of a slot across a plurality of data symbols, and in order to estimate a transmission line channel with a high degree of accuracy, a plurality of pilot symbols are used.
Accordingly, also in this instance, measurement of an SIR must be performed over a long interval (many slots) including at least two pilot symbol intervals between which a plurality of data symbols are held, and this is not suitable for an application in a fast fading environment. As a result, also a delay not only in SIR measurement but also in transmission power control is increased, and by such delay in transmission power control, the control error is increased and also the reception characteristic is deteriorated considerably.
Meanwhile, in the DS/CDMA communication system described above, also an application of an interference canceller which reduces interference from another user, which arises from a cross-correlation between diffusion codes, is an important technique.